Waterlaid leather substitute sheet and method for preparing the sheet

ABSTRACT

THE DISCLOSED SLURRIED SOLIDS COMPRISE BY WEIGHT: 2070% CATIONIC SOLIDS AND 80-30% ANIONIC POLYURETHANE LATEX SOLIDS, AT LEAST 30% OF THE CATIONIC SOLIDS BEING CHROME-TANNED LEATHER FIBERS AND AT LEAST 10%, PREFERABLY AT LEAST 25% OF THE CATIONIC SOLIDS BEING CATIONIZED WOOD FLOUR PARTICLES. THE SLURRY IS DEPOSITED ON A PAPERMAKING SCREEN IN THE CONVENTIONAL MANNER AND DRIED TO FORM SHOE INSOLE OR UPPER MATERIAL. HOT PRESSING CAN CONVERT THIS DRIED MATERIAL TO OUTSOLE MATERIAL.

United States Patent 3,756,909 WATERLAID LEATHER SUBSTITUTE SHEET AND METHOD FOR PREPARING THE SHEET Alfred H. Stepan and Robert J. Perkins, White Bear Lake, and Allen 1.. Griggs, St. Paul, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn. No Drawing. Filed Sept. 26, 1972, Ser. No. 292,330 lint. Cl. D21h 3/38 US. Cl. 162-151 12 Claims ABSTRACT OF THE DISCLOSURE The disclosed slurried solids comprise by weight: 20- 70% cationic solids and 80-30% anionic polyurethane latex solids, at least 30% of the cationic solids being chrome-tanned leather fibers and at least preferably at least 25% of the cationic solids being cationized Wood flour particles. The slurry is deposited on a papermaking screen in the conventional manner and dried to form shoe insole or upper material. Hot pressing can convert this dried material to outsole material.

FIELD OF THE INVENTION This invention relates to waterlaid sheets useful as leather substitutes and compositions and methods used to prepare them. An aspect of this invention relates to leather-like sheets made from fibrous material, a partial substitute for the fibrous material, and a polymeric binder, the sheets being made according to principles of papermaking technology. A further aspect of this invention relates to waterlaid sheets made from a furnish or slurry comprising leather fibers and an anionic polyurethane latex, the sheets being dried and, if desired, hot pressed after forming on the papermaking screen.

DESCRIPTION OF THE PRIOR ART Papermaking technology can provide well-controlled, high-production means for making waterlaid leather-like sheets from fibrous materials and polymeric binders. The sheets can be made to the desired thickness or in thin plies which can be laminated. See US. Pats. 3,436,303 (Raymond et al.), issued Apr. 1, 1969; 2,769,712 (Wilson), issued Nov. 6, 1956; and 2,601,671 (Wilson et al.), issued June 24, 1952. As demonstrated by the Raymond et al. patent, leather-like sheets made in this manner can have the properties of porous upper material as Well as sole (including insole) material depending on a variety of factors such as polymer-to-fiber ratios (see Example II of Raymond et a1.) and the amount of pressure used when heat-treating or densifying the sheet (see Examples VII(d) and VIII(B) (c) of Raymond et al.).

In various degrees of porosity and sheet or furnish formulations, waterlaid sheets of polyurethane latex binders and leather fibers have been found to have remarkable wear properties, particularly when chrome-tanned leather fibers are used. Unfortunately, the chrome tanning agents introduced into such sheets by the chrome-tanned fibers can detract from the ability of the sheets to be adapted to conventional upper, insole, outsole, and midsole manufacturing procedures. These procedures call for various machining and coloring operations; for example, in outsole manufacture, sheets are edge trimmed, heel scoured, edge-inked etc. Waterlaid urethane chrome-tanned leather sheets made according to the Raymond et al. patent and hot pressed to dense, substantially air-impermeable, outsole-like material (see Examples VII(d) and VIII(B) (c) of the patent) generate considerable heat (6065 C.) during cutting or edge trimming with conventional knives. A large part of this heat build-up appears to be due to 3,755,909 Patented Sept. 4, 1973 chrome tanning agents present on the leather fiber. To a lesser extent, heat may also be generated by the excellent wear properties of the sheet which interfere with ease of cutting, trimming, heel scouring, etc.

This heat build-up to 60-65 C. can occur in a few seconds and can dull knives, degrade the polymer in the sheet, and generally increase production time. Replacement of all or part of the chrome-tanned leather fiber with other fibers such as synthetic resin fibers, wood fibers, or vegetable-tanned leather fiber significantly alters the properties of the dried and heat-treated waterlaid sheet, necessitating a complete reformation of the furnish for the desired products and even redesigning the manufacturing steps. Even after reformulating the furnish and adjusting the papermaking process steps, it is still very difficult to match the properties of waterlaid sheets made with chrome-tanned leather and polyurethane latex solids.

Accordingly, this invention contemplates providing partial replacement for chrome-tanned leather fibers in the waterlaid sheet technology of Raymond et al., which replacement can facilitate adaptation of the sheet to shoe sole or upper manufacturing procedures without detracting the desired physical properties of the resulting sole or upper.

BRIEF SUMMARY OF THE INVENTION It has now been found that a waterlaid sheet comprising waterlaid cationic solids bound together with particulate anionic elastomeric polyurethane latex solids can be made by the previously described papcrmaking technology wherein the cationic solids compirse chrometanned leather fibers, but the resulting waterlaid sheet does not suifer the aforementioned disadvantage of heat build-up during machining steps. The disadvantages (e.g. the heat build-up) of using the chrome-tanned leather fiber are mitigated or substantially eliminated by replacing up to 70% by Weight of the chrome-tanned leather fibers with a special type of cationized wood flour. This replacement with wood flour can lower the density of the sheet and improve edge trimming, heel scouring, edge inking, and sole-laying of the waterlaid sheet without any significant sacrifice of the physical properties which were obtained when none of the chrome-tanned fibers were replaced. The ability to provide these advantages without loss of sheet properties is diflicult to explain in view of the truncated fiber bundles characteristic of wood flour. Other types of particulate wood are relatively more fibrous, e.g. wood pulp or ground wood, yet these more fibrous materials significantly increase the stiffness of the ultimately obtained sole or upper material and/ or significantly reduce the internal strength of the material. Surprisingly, the use of cationized wood flour according to this invention actually appears to improve some of the properties of the resulting outsole material, e.g. improved hand, reduced wet spread and bowing, and less telegraphing of defects underneath the outsole of a cement-processed shoe.

One preferred use of waterlaid sheets produced according to this invention is in the production of leather-like footwear materials. It is a feature of this invention that proper selection of the formulation or furnish and the processing conditions can provide a sheet with density, flex life, dry peelback resistance, tensile strength, stifiness, abrasion resistance, moisture regain characteristics, compression set resistance, water vapor permeability, etc., particularly suited for these uses wherein natural leather is presently used or even preferred, e.g. in the manufacture of shoe sole and upper material. The relative amounts of starting materials used in the preparation of waterlaid sheets of this invention are in part determined by the particular use contemplated. For example, an insole can comprise as little as 50 parts by weight anionic polyurethane latex solids combined with as much as 100 parts cationic solids, the cationic solids comprising chrome-tanned leather fibers and cationized wood flour. (The term wood flour is defined subsequently.) The methods of manufacture used in this case are substantially similar to the aforementioned Raymond et al. patent (No. 3,436,303), except for certain modifications found to be necessary to adapt this process to the use of cationized wood flour as a partial replacement for chrome-tanned leather fiber. Thus, the wood flour particles preferably are rendered cationic prior to being introduced into the papermaking-type slurry. Furthermore, since cationic solids are combined with anionic latex binder particles, it is preferred that flocculating agents of the type described in Raymond et al. not be used. Also, since the resulting material can be machined in the same manner as conventional outsole material, hot pressing of the dried Waterlaid sheet, as Example VII(d) or VIII(B) (c) of Raymond et a1. is actually a preferred practice.

DEFINITIONS The term waterlaid sheet has a well-understood meaning in the prior art (e.g. the aforementioned U.S. Pats. 2,769,712 and 3,436,303). The term denotes a paper-like sheet having a small thickness (relative to its area) and comprising solids which have been deposited from an aqueous slurry or the like onto a foraminous surface, e.g. onto the screen of a handsheet mold or papermaking machine. Papermaking machines can be of the Fourdrinier type, the cylinder type, the modified cylinder type, etc. It is also known to use a felt-like material as the forami nous surface. Typically, with these devices, an aqueous slurry (stock or furnish) is kept in suspension and at a desired consistency or solids content (e.g. 0.5% by wegiht) in a suitable container (e.g. a headbox or vat) such that the solids suspended in the slurry can be deposited on the foraminous surface. Removal of the water by vacuum and/or gravity provides a wet waterlaid sheet which can be dried and further processed.

The term wood flour is a term of art for a byproduct of the lumber industry and is also called sanding dust or wood meal. It differs from sawdust in that it is produced by an abrading rather than cutting or sawing operation. It differs from conventional ground wood or mechanical pulp primarily in that it is less fibrous and is a waste product, rather than a carefully selected or pretreated piece of wood ground into particles. Thus, neither wood flour or its unpulverized source has been subjected to any treatments such as bleaching. Wood flour typically comes in at least three grades of fineness, medium (e.g. 100

mesh) grades of hardwood flour, preferably maple wood flour, being preferred for use in this invention. Typical commercial grades of wood flour contains a small percentage of fine dust-like particles about 5 microns in size and a small amount of gross particles on the order of 400 to 500 microns in size, the major amount (by weight) of the particles being irregular chunks 50-425 microns in size, which, upon microscopic examination, appear to be truncated fiber bundles. Wood flour generally lacks the distinctly fibrous nature of mechanical or chemical wood pulp.

The term polyurethane and the term active hydrogen are used in this application substantially as in Raymond et al. US. Pat. 3,43 6,303. Thus, the polyurethanes used in this invention can contain thiourea or urea linkages in addition to or in lieu of urethane (carbamate) linkages.

The term anionic latex solids denotes particles less than about 25 microns in size which may or may not be agglomerates and are generally globular in shape. The anionic latex solids of this invention are preferably in the form of individual dispersoid particles having a numerical average size less than about 5 microns, but generally larger than about of a micron. The latex solids can be anionic by virtue of comprising negatively charged polymer molecules or can be emulsified with an anionic surfactant. The anionic (negative) charge of the latex particles helps to keep them dispersed in water (i.e. in a latex) through the formation of a phase relationship characteristic of oil-in-water emulsions.

The term cationic solids denotes solids particles which, at least at their surfaces, are characterized by one or more positive charges. These positive charges are typically provided by a treatment which alters the structure of the materials in the particle so as to create a positive charge or which adds a charge carrier such as a positively charged resin to the particles. A qualitative test for establishing the cationic nature of a suspension of particles in water will be described subsequently.

Chrome-tanned leather fiber is a readily available raw material, typically made from chrome-tanned leather scrap. The scrap can be ground or otherwise treated to provide fibers of papermaking length. Chrome-tanning is a term of the tanning art referring to treatments with conventional chromium tanning agents, which are salts or complexes of, for example, trivalent chromium.

The term elastomeric denotes the ability of an article, e.g. a cast film of polymer, to be elongated at least of its length and the return with force to substantially its original lenth. Elastomeric latex solids are those which can provide an elastomeric article when cast as a layer of aqueous dispersion, dried, and coalesced into a coherent film.

DETAILED DESCRIPTION AND EXAMPLES As pointed out previously, the anionic polyurethane latices used in formulating the furnish or papermakingtype slurry of this invention can be the polyisocyanate/ polyol and/or polyamine reaction products described in the Raymond et a1. patent (No. 3,436,303). The anionic character of the polyurethane latex solids can be provided by external anionic emulsifiers or surfactants (e.g. carboxylic, sulfonic, or phosphonic acid anions external to the polymer molecule). Alternatively, self-emulsified anionic latices wherein a carboxylic acid anion, a sulfonate, sulfate, phosphonate, phosphate, or the like is part of or substituted on the polymer molecule. See US. Pats. 3,539,483 (Keberle et al.), issued November 1970, and 3,479,310 (Dieterich et al.), issued November 1969, British specification No. 1,076,688, published July 1967, and German laid-open specification No. 2,053,900 (Carlson), laid open Apr. 29, 1971 for examples of self-emulsified anionic latices of the polyurethane type.

These anionic polyurethane latex solids, when formed into a film, exhibit the properties of elastomers described in the aforementioned 3,436,303 patent. As pointed out previously, the term polyurethane is used in the broadest sense so as to include polythiourethanes, polyurethane-polyureas, and other polyurethane-type polyureas, which are elastomeric and contain units, in the polymer chain, wherein R is a divalent aliphatic, aralkylene, or aromatic group such as an alkylene radical of 410 carbon atoms or a monocyclic or polycyclic aromatic or aralkyl nucleus such as benzene, toluene, xylene, diphenylmethane, naphthalene, etc.; X is O, S, NH, N-aliphatic, or the like; Z is a polyoxyalkylene or polyester chain; and Z is a divalent aliphatic, cycloaliphatic, or aromatic radical. Although these units are shown as divalent structure, it should be understood that, if a crosslinked, crosslinkable, or branched-chain polyurethane is desired, the Z Z or R groups can have one or more additional substituents. The Z radical is derived from a compound having the formula Z (XH) wherein Z and X are as defined previously, m is l-5, preferably 2 or 3, and H is an active hydrogen as defined previously; Z (XH) can be piperazine and the like.

In the preferred polymers, X is oxygen or NH. If the Z chains in the molecule are not the same, i.e. the polymer contains more than one kind of polyoxyalkylene and/or polyester chain, at least one Z chain preferably has a molecular weight of at least about 400 but less than about 5,000.

When Z is a polyester chain, the polyester units are preferably of the repeating formula wherein A and A are divalent aliphatic groups such as alkylene radicals. These polyester units can be derived from the interaction of a bifunctional initiator with one or more lactones, for example, as described in US. Pat. No. 2,933,477, or by an esterification or ester-interchange reaction involving a dicarboxylic acid or anhydride or ester thereof with an alkylene polyol, preferably an alkylene glycol.

When polyesters are prepared from dicarboxylic acids, anhydrides, or esters, and alkylene glycols, the preferred acid, anhydride, or ester, can be selected from a wide variety of polybasic (preferably diabasic) acids. It is preferred to use dibasic fatty acids such as malonic, succinic, or adipic acid. Examples of useful alkylene glycols are ethylene glycol; 1,3-propane-diol; 1,4-butane diol, and the like.

The stiffness of the polymer can be modified by introducing in the polyurethane-forming reaction various chain-extending, chain-branching, or chain-terminating agents, e.g. arylene diamine chain extenders. A' advantage of this invention is the freedom of using trior higher functionality materials to achieve crosslinking in the resultant finished sheet material. Preferred chain-branching agents are the triols and triamines commonly used in the polyurethane art. Chain propagation can be carried out in any suitable manner known in the art. Preferably, prepolymers are chain extended with polyols, polyamines, and/ or water. Chain propagation and emulsification of the polymer can be combined into a single step. Preferably, prepolymers are made from an aromatic diisocyanate, an active hydrogen component comprising a polyoxyalkylene glycol and an aromatic diamine, and, optionally, one or more compounds having 3*5 active hydrogen-bearing substituents (e.g. a triol), and, if desired, a suitable catalyst, e.g. an organo-metallic compound such as stannous octoate, mercuric acetate, phenyl mercuric acetate, or the like. Polyoxyalkylene diamines can be substituted for the polyoxyalkylene glycol with good results. An advantage of this substitution is that the resulting polyurethane (which is actually a polyurea) can be more degradation resistant. Carboxyl-containing compounds can be used in the active hydrogen components, but such use is not preferred.

The molecular weight, crosslink density (if any), amount of aromatic content (if any), amount of urea and/or urethane linkages, etc. of the polyurethane latex solids is selected such that the solids are elastomeric, as defined previously. Elastomeric polymers suitable for use in this invention have a molecular weight greater than 10,000 and form films with the following physical properties: (tested free of fillers and the like at 23 C. and 50% relative humidity) a tensile strength of at least 300 (21.1 kg./cm. p.s.i., preferably at least 750 p.s.i. (52.8 kg./cm. a stress at 100% elongation of at least 50 p.s.i. (3.5 kg./cm. preferably at least 150 p.s.i. (10.5 kg./cm. and an elongation at break of at least 300%, preferably at least 500%. To avoid undue stiffness, the stress at 100% elongation should not exceed 1000 p.s.i. (70 kg./cm. To avoid undue rubberiness, the elongation at break should not exceed 00%.

As pointed out in the aforementioned 3,436,303 patent, polyurethanes useful in this invention generally have been found to have a brittle temperature of about 10 C. or lower, and preferably minus 30 C. or lower. The heat distortion temperature has been found to be at least +40 C. or preferably at least C.

By following the teachings of the self-emulsified polyurethane latex art, latices comprising particles less than a few microns in size can readily be provided. It is difiicult, however, even with special mixing or homogenizing equipment, to reduce all of the particles to the colloidal size range. This presents no serious problem, however, since charged polyurethane latex particles larger than 0.1 micron in size form stable oil-in-water emulsions. Uncharged polyurethane particles in this size range can be stably emulsified with the use of suitable anionic surfactants. In any event, the anionic latex solids used in this invention serve as a binder for the cationic solids described subsequently.

The cationic solids of this invention comprise a mixture of tanned leather fibers and suitable Wood flour particles which have been rendered cationic, preferably with a water-compatible cationic resin. A simple qualitative test can be used to determine whether or not a given sample of particulate material is cationic in the sense of this invention. It is known that very finely divided anionic clay can be dispersed in water with the aid of a dispersing agent to provide a milky suspension with a slow rate of sedimentation. A pH slightly on the alkaline side can be used to insure that the clay particles remain anionic. If a substantially neutral or anionic material is added to the aqueous clay suspension, the sedimentation rate of the clay is not significantly affected, even if the substantially neutral or anionic particles themselves settle rapidly. On the other hand, if a sample of cationic particles large enough to settle in water is added to the aqueous clay suspension, the rapidly settling cationic particles bring the anionic clay particles down with them, thus significantly increasing the sedimentation rate of the clay. The conditions of the test will be described in detail in the examples which follow.

The wood flour used in this invention has been defined previously in general terms. This material is the byproduct of an abrading operationtypically an operation of the type wherein an unshaped piece of wood is sanded, rather than cut or planed, to the desired shape. Commercially available wood flour is classified in three rough grades: 40 mesh, mesh, and mesh, the mesh sizes referred to being either US. or Tyler. The 40 mesh grade is rich in relatively gross particles, 250-425 microns in size, which barely pass through a 40-mesh screen and would ordinarily be retained on a 60-mesh screen. The 100 mesh grade is an approximate term, since it includes particles in the 225-425 micron range and does not completely exclude all particles retained on a l00-mesh screen (theoretically all particles larger than about microns are retained on a l00-mesh Tyler or US. screen). However, the 100 mesh grade is considerably richer than particles 50225 microns in size, this fraction being the major amount, eg 75%, by weight of the wood flour. The l40-mesh grade is especially rich in the very fine, dust-like wood flour particles 5-110 microns in size, most of the grosser particles having been screened off. It has been found that the wood flour fines, i.e. the dust-like particles as small as 5 microns in size, tend to alter the properties of water laid sheets made with chrome-tanned leather fiber and anionic polyurethane latex solids. The grosser particles, if properly rendered cationic, do not interfere with the properties of sheets made according to this invention. However, particles large enough to be retained on a 40-mesh screen, or even particles larger than about 250 microns in size, tend to distort the appearance and feel of sheets made according to this invention. Accordingly, it is preferred that these grosser particles be present in minor amounts in the cationized wood flour used in shoe outsole or upper material made according to this invention. When sheets made according to this invention are used in products wherein appearance is not critical, e.g. as abrasive backup pads,

larger amounts of the grosser particles are acceptable. Microscopically, the larger wood flour particles used in this invention have the appearance of truncated fiber bundles. The fibrous nature of the wood is apparent in these particles, but due to the effects of abrasion, the wood is broken down into irregular chunks rather than fibers. A microscopic comparison of mechanical or chemical wood pulp and wood flour strikingly reveals the relatively non-fibrous character of wood flour. Thus, it is surprising that the wood flour functions better in products of this invention than does wood fiber; it would be expected that leather fiber should be replaced with some other type of fiber rather than with a filler-like material. The very finely divided, dust-like particles of wood flour, particularly those about to about 50 microns in size, appear to behave more like wood fiber (and have the same detrimental effects upon the sheet). This phenomenon has no readily apparent explanation, but, in any event, it is preferred to remove or minimize the amount of these fines in the wood flour of this invention. Accordingly, the

140 mesh classification is not preferred, i.e. the amount of 5-50 micron fines is preferably kept below by weight. Accordingly, the 100 mesh classification provides the best results in this invention.

The wood flour preferred for use in this invention is preferably rendered cationic with a water-compatible cationic or cationizable resin. By water-compatible is meant a resin which will form a true solution or a stable dispersion in water. Cationic water-compatible resins have been used in the preparation of waterlaid sheets for many years, and typical dimethylolurea water-soluble polyfunctional nitrogen base resins selected for this purpose are described in the aforementioned Wilson patent, U.S. Patent 2,769,712. The previously mentioned Wilson et al, patent, U.S. Patent 2,601,671, discloses cationic melaminealdehyde condensation products also useful as cationic resins which impart a positive electrical charge to leather when absorbed on the surface of leather fibers. In the present invention, it is preferred that the wood flour be treated with a polyamide-epichlorohydrin wet-strength resin of the type described in TAPPI, 52, pages 1157-1161 and 1162-1168 (1969). (TAPPI is the technical journal of the American Pulp & Paper Industry.) As pointed out in the two TAPPI articles, commercially available polyamide-epichlorohydrin (PAE) resins, such as Kymene 557 (trademark of Hercules Powder Company), comprise the reaction product of epichlorohydrin with the condensation polymer obtained from adipic acid and diethylenetriamine. Cationic sites in the polymer molecule are formed primarily by conversion of the secondary amine to quaternary nitrogen by alkylation with the epichlorohydrin. Some tertiary amines and very small amounts of primary and secondary amine groups are also typically present in the resin. In appearance, Kymene 557 (trademark) is a pale amber liquid with a solids content, in water, of 9-11%. As manufactured, it has a viscosity at C. of -65 centipoise and a pH of 4.6- 4.9. Its freezing point is 30 F. and its nitrogen content (dry basis, Kjeldahl) is 12.8%.

Melamine-formaldehyde and polyethyleneimine cationic polymers are also commercially available, as is pointed out in the TAPPI articles.

To be useful in this invention, it is preferred that the 100 mesh wood flour be first slurried in water and then treated with the cationic wet-strength resin prior to being combined with the leather fiber and/ or anionic polyurethane latex in the furnish.

A waterlaid sheet of this invention can be formed from a mixture of the cationic solids, comprising cationic leather particles and cationized wood flour, and an appropriate amount of anionic elastomeric polyurethane latex solids. The amount of anionic latex solids can vary, depending upon the end use of the waterlaid sheet and the properties needed for that use. For permeable shoe upper material, the amount of anionic latex solids should be at least equal to the amount of cationic solids and can be as high as by weight of the total solids. At the other extreme, loose, porous insole material can be made from as little as thirty parts by weight of anionic latex solids and 70 parts by weight of the cationic solids. For outsole material, the preferred proportions are similar to those of upper material, the upper material being, in effect, an intermediate in the production of outsole-type material. As is shown by Raymond et 211., hot pressing of the upper material drastically reduces its porosity and densifies it, thus providing the characteristics of the tough, substantially impermeable material from which outsoles are made. As pointed out previously, the leather fiber is cationic by virtue of chrome-tanning and the wood flour can be rendered cationic with a suitable cationic resin or the like. It is permissible to include other materials in the papermaking furnish such as synthetic staple fibers of papermaking length and vegetable tanned leather fibers. The electrical charge characteristics of the leather fiber can be adjusted, if desired, with suitable cationic resins, as is shown by the aforementioned Wilson and Wilson et al. patents. For shoe sole and upper materials, however, it is preferred to use substantially all chrome-tanned leather fibers, which can be obtained as a byproduct from the chrome-tanning of leather hides and which do not require any special additional treatment to render them cationic.

As in the case of the overall cationic solids/anionic solids ratios, the ratios of chrome-tanned leather fibers to cationized wood flour particles are subject to variation depending upon the desired properties for the ultimately obtained waterlaid sheet. For outsole material, the total amount of cationic solids should comprise less than about 55% of the waterlaid, dried, and pressed sheet, and no more than about 70% by weight of this 55% component should be wood flour, thereby insuring that at least about one-sixth by weight of the outsole will comprise leather fiber.

The introduction of wood flour into sheets made according to this invention does not detract from the flex life, dry peelback resistance, tensile strength, stiffness, resistance to wet spread, etc., that the sheet would otherwise have if no wood flour were used. The density of the sheet is lowered slightly by the wood flour, and this can be advantageous in some contexts. The abrasion resistance is also lowered, but this can be an advantage in that it facilitates machining of the outsole material. Virtually no lowering in density is noticeable when less than 10% of the cationic component of the slurried solids in the furnish is wood flour. The various advantages of introducing wood flour become more apparent as the percentage is increased to 20% by weight. When more than 25% by weight of the aforementioned cationic component is wood (flour, some of the most difficult problems of machining, e.g. heat build-up during edge trimming of outsole material, become noticeably alleviated. It is not necessary to increase the wood flour content of this cationic component beyond about 55 by weight of wood flour to substantially mitigate or eliminate edge trim problems. As pointed out previously, wood flour is most effective in eliminating edge trim problems when its source is a hard wood, such as maple. As is known in the art, hard woods and soft wood differ in their relative amounts of cellulosic materials and lignins as well as their cellular structure. For example, hard woods such as maple are generally lower in mannan and galactan than are soft woods. On the other hand, hard woods tend to be higher in xylan. The reasons why hardwood flour performs better than softwood flour in this invention are not understood, however.

Outsole and upper materials can be made according to this invention with a dry peelback value, determined at a peel rate of 12 inches per minute, of at least 12 lbs. per lineal inch, an apparent density in the range of 0.7- 1.2 grams per cc., preferably 0.9-1.1 g./cc. for outsole material, and a Ross flex life (as defined hereinafter) of at least 30,000, preferably at least 50,000 cycles. The Ross flex life of sheets of this invention is determined by cutting a hole in a sample sheet and flexing it on a Ross Rubber Flexing Machine (Emerson Apparatus Company, Melrose, Mass). Samples are conditioned at 80% relative humidity and 2025 C. before the Ross flex test.

The aforementioned dry peelback resistance should be at least lbs. per lineal inch for even a marginally useful outsole material. A good outsole material should have a dry peelback resistance of at least 12 lbs./in., preferably lbs/in. The term dry peelback resistance is sometimes referred to as internal bond strength.

A simple relative measurement of stiffness can be obtained by securing three inches of a six-inch sample of material on a hard surface or in a set of jaws and applying weights to the free end of the sample. By this test, one-half by Weight of the leather fiber can be replaced by wood flour in accordance with the teachings of this invention with no undue increase or decrease in stiffness. The percent of spread of the sheet, when the sheet is wet, is actually reduced by using wood flour in accordance with this invention. This reduction in wet spread is beneficial in outsoles.

The method of manufacturing waterlaid sheets of this invention is similar to the method described in the aforementioned 3,436,303 patent. One difference is that the critical additive of this invention, wood flour, is rendered cationic before being included in the papermaking-type slurry. Furthermore, since this invention contemplates using primarily cationic leather fiber, the aforementioned cationic wood flour, and anionic latex solids, the latex solids tend to be fiocculated or precipitated out onto the cationic solids without the use of any special flocculating agents, although the pH can be controlled by one or more of the agents described in U.S. Pat. 3,436,303, column 6, line 51, et seq. The flocculating of the anionic latex solids onto the cationic solids can occur in the headbox or vat of the papermaking machine. For laboratory samples made on hand sheet molds, the slurry can be made up and flocculated in a beaker. An oil-in-water anionic latex with a solids content ranging from 1 to about 70 wt. percent (or to just below the inversion point) can be added directly to the slurry with or without dilution, the preferred solids content being 10- 50 wt. percent. After deposition of the solids on the screen and draining off of the water, the sheet can be dried in a hot-air oven or with a heated element such as a hot platen or roll. Drying of the,

sheet produces a tough, coherent material which canbe further toughened with heating at temperatures above 100 C. As pointed out in the 3,436,303 patent, mild pressure, even at moderately elevated temperatures, causes the the polyurethane in the sheet to fiow-a process which is detrimental to shoe upper material, but which can be advantageous for outsoles. This flowing of polyurethane o'bliterates the individuality of flocculated polyurethane particles or particle clusters and raises the Gurley densimeter value well above 2,000 seconds, but high Gurley values are desirable for outsoles.

Sheets can be waterlaid to any desired thickness, ranging from about 10 mils to 400 or 500 mils, which would be even thicker than a 12-iron shoe sole. When it is not convenient to form 6-iron, 9-iron, or l2-iron sheets to size on the papermaking machine (a 6-iron sheet is 0.32 cm. thick, and a 9-iron sheet is 0.48 cm. thick), relatively thin sheets can be formed and laminated with heat and pressure and/ or adhesives, as described in US. Pat. 3,436,303. The dried, heat-treated and/or pressed product can be finished in the conventional manner to give it the finished appearance of upper or outsole material.

For outsole material, at least 40% by weight of the cationic leather fiber should be replaced by cationic wood flour of the appropriate grade of fineness.

Although the preceding description has been directed primarily toward using the waterlaid sheets of this invention as elements of footwear, other uses will occur to the skilled technician. Generally, any use wherein a tough, leather-like sheet is needed is an area of utility for products of this invention. Examples of such uses include pads for abrasive-surface discs, backup layers for diecutting, gaskets, furniture, conveyor belts, and the like. As elements of footwear (e.g. shoes and boots), material produced according to this invention can be made into midsoles and heels as well as insoles, outsoles, and uppers.

When polyamide-epichlorohydrin cationic resins are used to render the wood flour cationic, acceptable levels of cationization have been obtained with 0.3% by weight of the resin solids, based on the weight of the wood flour. It is preferred to use at least 0.5% by weight of the cationic resin, and amounts up to 2% are useful. Amounts of water soluble cationic resin in excess of two weight percent do not add to the performance of the resulting waterlaid sheet and may act as an unnecessary diluent. Larger amounts of water dispersible cationic resins can be used, however.

In the following non-limiting examples, all parts and percentages are by weight, unless otherwise specified.

EXAMPLE 1 Cationic treatment of wood flour (A) Wood flour of the so-called 100 mesh grade obtained by sanding of maple was found to consist of small dust-like particles mixed with truncated fiber bundles up to 425 microns in size. The particle size distribution was as follows:

10 wt. percent less than 57 microns in size, 50 wt. percent less than 142 microns in size, and wt. percent less than 225 microns.

Five parts by weight of the wood flour were slurried in ml. of water and 5 ml. of a 1 wt. percent aqueous solution of cationic polyamide-epichlorohydrin (PAE) resin (Kymene 557, trademark of Hercules Powder Co.) was added. The wood flour became cationic, as was demonstrated in the clay sedimentation test.

(B) A clay sedimentation standard was prepared from ml. of deionized water and 0.5 gram of kaolin-type clay (Hubers Hi-Wite R [trademark]). The Hi- Wite (trademark) is known to be anionic throughout the pH range of 4.58.0 and is known to have an average particle size of 1.5 microns. The resulting aqueous clay suspension, with gentle stirring, was a milky suspension with a slow sedimentation rate.

The sample to be tested consisted of 1.0 gram of material, which was slurried in 150 ml. of deionized water. The particle size range of the test sample was primarily above 50 microns so as to insure rapid settling of sample particles, and these relatively large particles were kept in suspension with stirring. To this suspension was added 0.5 gram of the previously described kaolin-type clay. The stirring of the sample was stopped shortly after the clay was added. If the sample was cationic, the sedimentation rate was noticeably faster than the standard, by a factor of about 100. If the sample was not cationic, the sedimentation rate of the clay was similar to that of the standard.

A sample of the PAE-treated wood flour of this example was found to be cationic by the above-described clay sedimentation test. A sample of the so-called 100- mesh grade maple wood flour not treated with the PAE resin was found to have no visible effect upon the clay sedimentation rate, despite the fact that 100-mesh grade particles settled rapidly in water. Differences in the character of sedimentation were also observed. Microscopic examination of the PAE-treated wood flour particles revealed specks of clay on the particles. Similar examination of the untreated wood flour revealed that this material did not have any observable attraction for the anionic clay particles. Thus, the wood flour rendered cationic according to Part (A) of this example was ready for use in a furnish for a leather/polymer Waterlaid sheet of this invention.

EXAMPLE 2 Waterlaid sheets (A) For comparison purposes, a Waterlaid sheet was prepared substantially according to Raymond et al., U.S. Pat. 3,436,303, except that the latex used was a sulfonated polyurethane latex made substantially according to Example 3 of laid open German specification No. 2,053,900 (Carlson), published Apr. 29, 1971. The formulation of the solids used in the papermaking-type slurry was:

Parts by weight Chrome-tanned leather fiber (see Example I of U.S. Pat. 3,436,303) 100 Sulfonated polyurethane-urea solids (see Example 3 of German spec. 2,053,900, 40 wt. percent solids 100 *Based on pure leather with solubles, etc. leached out.

The latex solids were permitted to exhaust or flocculate onto the chrome-tanned leather fibers by virtue of the difference in electric charge.

The resulting Waterlaid sheet was 300 mils in caliper. After drying in a circulating forced air oven at ZOO-240 F. ambient temperature, a permeable sheet was obtained. The permeable sheet was densified with 320 F. of heat and 125-140 p.s.i. pressure for 3.5 minutes. (The sheet will withstand 320 F. for several minutes without undergoing any degradation of the polymer, leather or wood flour.) A 190-mil (about 9 iron) outsole-like material was obtained, which, after suitable conditioning, was found to have a density of 0.96 g./cc., a 180 dry peel resistance of 15 pounds per lineal inch. In the Ross flex life test, no failure was noted at 50,000 cycles, the tear propagation being from 0.1 inch to 0.2 inch.

(B) A Waterlaid sheet was made according to this invention by pretreating maple wood flour in accordance with Example 1(A) with 0.5%, by weight, of the wood flour, of PAE resin solids (Kymene 557). A wt. percent aqueous slurry of the cationized wood flour was added to 5 wt. percent aqueous slurry of chrometanned leather fiber, and the two slurries were thoroughly blended. The latex of Example 2(A) was added to the combined slurries, and the resulting final combination of slurries was diluted to a papermaking consistency (2% solids by weight). The amounts of the various slurries were controlled so as to provide a furnish containing the following amounts of solids:

Component (see Part A of this example): Parts by weight Chromed-tanned leather fiber 55 Cationized wood flour 45 Polyurethane-urea solids 100 Conventional surfactant-type anti-foaming agents can be added to reduce foaming in the headbox.

The resulting Waterlaid sheet was dried and hot pressed to a 190-mil outsole material as in Example 2(A). Properties of the outsole material were:

Density: 0.96 g./cc. Dry peel back resistance: lb./lineal inch Ross flex life: 50,000 cycles *The observed tear propagation was from 0.1 inch to 0.5 inch, but the sample had not failed.

In both Parts A and B of this example, the heated and pressed sheets were substantially impermeable to air. As shown by the 3,436,303 patent, Example VII(d), the Gurley densimeter value for a heated and pressed sheet is above 2,000 seconds per 400 cc. of air per 60 mils thickness. Microscopic examination of the heated and pressed sheets revealed that the particulate structure of the polyurethane latex solids had been obliterated.

12 EXAMPLE 3 Waterlaid insole material The procedure of Example 2(B) was followed, except that the solids formulation for the furnish was as follows:

Component (see Example 2): Parts by weight Chrome-tanned leather fiber 50 Cationized wood flour 50 Polyurethane-urea solids 50 The resulting sheet was useful in the manufacture of insoles for Oxford-type shoes or boots.

What is claimed is:

1. A Waterlaid sheet comprising Waterlaid cationic solids bound together with particulate anionic elastomeric polyurethane latex solids, said sheet comprising, by weight: 20-70% of said cationic solids, -30% of said particulate anionic elastomeric polyurethane latex solids, about 30 to about by weight of said Waterlaid cationic solids being chrome-tanned leather fibers of papermaking length, and about 10 to about 70% by weight of said Waterlaid cationic solids being wood flour particles rendered cationic at least on their surfaces, the major amount by weight of said wood flour particles being 50-425 microns in size.

2. A Waterlaid sheet according to claim 1 wherein said wood flour particles are hardwood particles about 5 to about 425 microns in size having the following particle size distribution:

10% by weight less than 57 microns in size, 50% by weight less than 142 microns in size, and 90% by weight less than 225 microns in size.

3. A Waterlaid sheet according to claim 1 wherein said sheet has been subjected to sufficient heat and pressure to cause the particles of said particulate anionic elastomeric polyurethane latex solids to flow and fuse together, obliterating boundaries between particles, said sheet having a Gurey densimeter value greater than 2000 seconds per 400 cc. of air per 60 mils of sheet thickness, said Waterlaid sheet having a dry peel back, determined at a peel rate of 12 inches per minute, of at least 12 pounds per lineal inch, and a Ross fiex life of at least 30,000 cycles.

4. A Waterlaid sheet according to claim 1 wherein said wood flour particles have been rendered cationic by means of a treatment with a cationic resin.

-5. A Waterlaid sheet according to claim 4 wherein said cationic resin is a quaternary nitrogen-containing polymer of the polyarnide-epichlorohydrin type.

6. A Waterlaid sheet according to claim 1 wherein said Waterlaid sheet comprises less than 55% by weight of said cationic solids and comprises at least about one-sixth by weight of said leather fibers.

7. A Waterlaid sheet according to claim 1 wherein 25- 55% by weight of said cationic solids comprise said wood flour particles.

8. A Waterlaid sheet according to claim 3, said Waterlaid sheet being 10-500 mils thick and comprising less than 55% by weight of said cationic solids; at least 40% by weight of said cationic solids being said wood flour particles and at least one-sixth by weight of said sheet being said leather fiber.

9. A Waterlaid sheet comprising 20-55% by weight Waterlaid cationic solid particles bound together with 80- 45% .by weight of particulate anionic elastomeric polyurethane latex solids fiocculated onto the surfaces of said cationic solid particles; said cationic solid particles comprising leather fibers and cationic wood fiour particles, about 25 to about 55% by weight, based on the total weight of said cationic solid particles, being said cationic wood flour particles, the major amount by weight of said cationic wood flour particles being larger than 50 microns in size; at least one-sixth by weight of said waterlaid sheet being said leather fibers.

10. An aqueous slurry comprising a major amount of water and, suspended therein, --70% by weight of cationic solids slurried with 80-30% by weight of particulate anionic elastomeric polyurethane latex solids, said anionic elastomeric polyurethane latex solids being flocculated onto said cationic solids, about to about 90% by weight of said suspended cationic solids being tanned leather fiber of papermaking length, and about 10 to about 70% by weight of said suspended cationic solids being wood flour particles rendered cationic at least on their surfaces, at least a major amount by weight of said wood flour particles being -425 microns in size.

11. A method for making a waterlaid sheet from cationic wood flour particles, cationic leather fibers of paperamking length, and particulate anionic elastomeric latex solids comprising the steps of:

(1) suspending about 10 to about parts by weight of said cationic wood flour particles and about 30 to about parts by weight of said cationic leather fibers in a major amount of water to form an aqueous slurry,

(2) adding to said slurry an amount of aqueous anionic elastomeric polyurethane latex sufficient to make the resulting solids content of said slurry 30- 80% by weight particulate anionic elastomeric polyurethane latex solids,

(3) flocculating said anionic elastomeric polyurethane latex solids onto the surfaces of said cationic wood flour particles and said cationic leather fibers while maintaining said wood flour particles and leather fibers in suspension in said aqueous slurry,

(4) depositing the aqueous slurry of Step (3) onto a foraminous surface which retains the solids of said slurry while permitting the water of said slurry to drain otf from the said solids, and

(5) drying the resulting waterlaid sheet.

12'. A method according to claim 11 wherein:

said wood flour particles are rendered cationic by treating them with a cationic resin while maintaining the wood flour particles suspended in water;

said Step (1) of said method is carried out by combining a separate aqueous slurry of wood fiour with a separate aqueous slurry of leather fiber;

the total solids content of the aqueous slurry of said Step (3) is adjusted to 0.5-5% by weight; and the dried waterlaid sheet obtained from said Step (5) is heated to a temperature greater than C. and subjected to suflicient pressure for a sufficient period of time to cause said elastomeric polyurethane latex solids to flow and to cause the particulate nature of said latex solids to be obliterated.

References Cited UNITED STATES PATENTS 1,305,770 6/1919 Clapp 162 142 2,601,671 6/ Wilson 162-443 3,436,303 4/1969 Raymond 162144 2,104,996 1/193'8" Ives 162-144 S. LEON BASHORE, Primary Examiner P. CHIN, Assistant Examiner U.S. Cl. X.R. 162-158, 168 

